Moving source dipole electromagnetic exploration device for deeper and poorer conductors and a method of detecting such conductors

ABSTRACT

The present invention relates to moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies the present invention also relates to a method for detecting deeper and poorer electrically conducting subsurface bodies.

FIELD OF THE INVENTION

[0001] The present invention relates to moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies. The present invention also relates to a method for detecting deeper and poorer electrically conducting subsurface bodies.

BACKGROUND AND PRIOR ART REFERENCES

[0002] The constant search for new mineral deposits have become important to our civilization as the demand for metals, petroleum and water increase with increasing industrialization. Geophysical methods which take advantage of the various physical properties of the earth's material, have a well-established place in this under-ground exploration. Electrical conductivity, a very wide-ranging physical property of 10⁻¹⁴ to 10⁸ Sm⁻¹, has ben exploited in electromagnetic methods of exploration. Electromagnetic methods fall into two categories:

[0003] natural field methods, and

[0004] (ii) controlled-source methods

[0005] The first method relies on source fields generated by ionosphere and magnetotelluric current or spontaneous current associated with electrochemical activity of the earth's material. As the source distribution in the natural field methods is known, the number of a variables increases. In the method artificial field can be created by means of direct, alternating or pulsed, current. The energizing source is under control of user and can be exploited to increase the resolution of data by using variable frequencies. Artificial fields can be applied conductively (through electrodes) or inductively (by means of coils or large wire loops). Finally, in both the methods different parameters of the resultant electric or electromagnetic field are measured. Electromagnetic induction methods are based on the well known principle of electrical secondary currents flow in such a way that the resulting electromagnetic field opposes the primary inducing field. The two fields will have the same frequency but will generally differ to direction, magnitude and phase with resultant field being elliptically polarized. A number of exploration systems have been conceived on the basis of geometry of plane of polarization. The secondary electromagnetic field is commonly termed in geophysical literature as the response or the anomaly due to the target. Its magnitude and variation in space and time also comprises the basis of some of electromagnetic systems. The anomaly characteristics are suitable interpreted in terms of geological and geometrical characteristics of the causative body.

[0006] In mineral prospecting, inductive electromagnetic methods have attained greater popularity than conductive electromagnetic methods as the former react to absolute conductivity, rather than conductivity contrast and can distinguish between highly and moderately conductive target. Also, in large regions of permafrost, deserts or artifacts where conductive contact is not possible due to high resistivity layer on the surface inductive methods can be employed conveniently. An additional advantage of them over conductive methods is that they do not require electrical contact with the ground and thus can be moved rapidly over earth's surface and also adopted in airborne reconnaissance survey.

[0007] Usually inductive electromagnetic methods are further classified according to the type of energizing source the receiving system employed and the quantity measured. The methods the distance between source and target is variable, giving rise to larger anomalies. Portability requirements dictate that in most instances the source should be three-dimensional (3D). On the other hand, in fixed source methods the distance between source and receiver is variable and is fixed between source and target. The power of the source can be increased considerably which is not possible beyond a certain limit with the moving source methods.

[0008] Artificial moving source inductive Electromagnetic (EM) methods in frequency domain are widely employed as they can be used in field prospecting both as ground and airborne survey.

[0009] Most commonly used transmitter [T]—receiver [R] coil configurations are: (1) Horizontal—Horizontal [Horizontal Coplanar]; (2) Vertical—Vertical—[Vertical Coplanar]; (3) Vertical Co-axial and (4) Horizontal—Vertical—[cross coupled], Horizontal Coplanar method has achieved steadily increasing popularity since its introduction in the late 1930s. This method, also know as Slingram, employs a variable frequency (100 Hz-4000 Hz) and separation (20 m-200 m). A systematic nomenclature for various moving transmitter—receiver configurations is given by Parasnis (1970). The quantities measured are inphase (IP) and quadrature (OP) (90° out of phase) components of the anomaly vector resolved in time phase with respect to the primary field.

[0010] Phase measuring techniques carry other advantages besides improved accuracy in locating anomalies. Firstly phase difference between the primary and resultant field is essentially a target conductivity phenomenon, which is not affected by geometric irregularities in the primary field. Secondly, the phase provides a clue in the conductivity of the target. Because of essential simplicity of instrumentation, operation and the progress in electronics in phase measuring techniques this method received further impetus both in direct application from air and in subsequent ground follow up. In a field survey the profile directions are laid perpendicular to the strike of the conductor. Two men crew is sufficient to conduct a survey. Survey can be done in INLINE configuration and BROADSIDE configuration.

[0011] The Transmitter [T] and Receiver [R] are moved in tandem along the profile direction in inline configuration. In this configuration T and R straddle the target one after other. In Broadside configuration the T and R are held parallel to the strike of the conductor and perpendicular to profile direction.

[0012] In frequency domain several EM mineral prospecting moving source-receiver systems have been fruitfully used for nearly six decades to detect shallow massive sulfide ore bodies. However, the ratio of the feeble response from the target to the strong primary field (which is the parameter measured with these systems) is very small in case of deeper and poorer conducting bodies.

[0013] Time domain EM methods were, therefore, developed wherein a repetitive pulsed primary field energizers the earth and the transient secondary field is measured while the primary field is zero. However, these methods require more intense primary field and the circuit design for transient signals is more difficult than for sinsoidal signals.

[0014] Frequency domain methods, however, are more suitable for faster reconnaissance and detail, are lightweight, portable, cheaper units and probably produce better anomaly resolution. They also have better developed data interpretation. The depth of exploration for all moving dipolar source methods is only 0.6-0.8 L (L=T−R SEPARATION).

OBJECTS OF THE INVENTION

[0015] The primary object of the present invention is to provide a moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies.

[0016] Another object of the present invention is to provide a method for detecting deeper and poorer electrically conducting subsurface bodies.

[0017] Yet another object of the present invention is to provide a device for exploration of mineral, ground water, archaeology and mapping of subsurface geology.

[0018] Yet another object of the present invention is to provide a device with transmitter coil placed strategically that is not affected by the primary field and measures only the secondary field.

SUMMARY OF THE INVENTION

[0019] The present invention relates to moving dipole source electromagnetic induction device for deeper and poorer electrically conducting subsurface bodies. The present invention also relates to a method for detecting deeper and poorer electrically conducting subsurface bodies.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention is further explained in the form of following embodiments. Transverse component of the time varying EM field surrounding a circular transmitter coil (T) is zero along same axes. If receiver coil (R) is located at these strategic points parallel to the T, currents are not induced in it by the primary field. Since any induced currents are then the only representative of the secondary field and can be measured with greater accuracy, in absence of the strong primary field.

[0021] The transverse component Hz of the magnetic field (perpendicular to the plane of the transmitter coil) due to a current 1 in a circular coil of radius ‘a’ is given by (Telford et al. 1976) $H_{z} = \frac{\text{?}\left( {{2Z^{2}} - L^{2}} \right)}{4\left( {Z^{2} + L^{2}} \right)^{5/2}}$ ?indicates text missing or illegible when filed

[0022] where Z and L are the Cartesian coordinates of the point of measurement with reference to the center of the coil. Hz is zero if

[0023] 2Z²−L²=0

[0024] i.e. at Z=±0.7071 L

[0025] Thus if the receiver R and transmitter T are separated laterally by L and vertically by Z −0.0701 L with reference to the plane of the coils, then the primary field of T will not induce any current in R. This property is clearly independent of the magnitude and frequency of the current in the transmitter coil. Further the locations of transmitter and receiver coils are inter changeable. If the two coils are placed horizontally, however, one of the coils should be at least at a height of 0.7071 L rendering such a field system unwieldy.

[0026] This limitation can be overcome if the coils are deliberately placed vertically.

[0027] It should be specifically mentioned that the equipment can be used in any conventional setup i.e., T-R coils can be placed in any configuration.

[0028] Secondly, the amplitude of the secondary field is measure (not only the IP and OP) which gives us the advantages (a) detection and measurement of feeble secondary field, in absence of primary field, yields comparatively, larger anomalies and hence makes it possible to detect poorer and deeper conductors, (b) by increasing the transmitter power, the anomalies are increased. This is not possible in the conventional IP/OP measurement because they are recorded as percentage of the primary field.

E.M. Modeling Apparatus

[0029] A frequency domain electromagnetic moving source apparatus has been developed for laboratory model studies. Sampling technique has been used to measure in-phase (IP) and out-of-phase (OP) components of anomalous field. The phase reference is with respect to transmitter current.

[0030] The development of the equipment can be broadly classified into two field (i) hardware and (ii) software. The hardware is further divided into [i(a)] analog hardware and [i(b)] digital hardware.

[0031] [i(a)] Analog Hardware

[0032] The analog hardware shown in FIG. 1 includes electronics assembly of transmitter and receiver sections of the system.

[0033] The oscillator (1) generates variable frequency from 1 kHz to 100 kHz sinusoidal signal with constancy of signal amplitude and frequency.

[0034] The output is fed to power amplifier (2). The power amplifier is commercially available with 10 watts assembly board.

[0035] This output signal drives a transmitter coil (3) to generate primary EM field. The transmitter coil has self-inductance £=0.95 mH; SWG=30 ; diameter=5×10⁻³ m and number of turns=650.

[0036] The tuning capacitors (4) comprise of an array of capacitors that resonantly tune the transmitter coil to achieve maximum output at a desired frequency. The tuning capacitors resonantly tune the transmitter at frequency to effectively cancel the impendances due to inductance as follows: $f_{c} = \frac{1}{2\quad \Pi \left. \sqrt{}{\mathcal{L}C} \right.}$

[0037] Where f_(c) is tuning frequency. £ is inductance of transmitter coil and C is capacitance of the tuning capacitors.

[0038] One nearly in phase (IP) sinusoidal signal with the transmitting current is obtained from the series resonant circuit. The IP signal is fed to a capacitor network (5) to obtain nearly out of phase (OP) sinusoidal signal. These IP and OP signals are fed to tune separate amplifiers (6 a, 6 b).

[0039] These amplifiers are developed with high input impedance field effect transistor (FET) operational amplifiers having gain of 10.

[0040] The two amplified signals are fed to the inputs of a dual comparator (8). The other inputs of the comparators are at steady dc voltages. The dc voltages are derived through two multiturn potentiometers (7 a, 7 b).

[0041] With the sinusoidal voltage at one of its input and steady dc on the other, the comparator produces a rectangular waveform.

[0042] The sampling pulse generator (9) consists of two monostable multivibrators. One of the two transitions of the comparator output is used to trigger mono-stable multi-vibrator which generates 2 μs short duration sampling pulse. The steady dc voltage level controls are adjusted to obtain proper instance of sampling for IP and OP component measurements. The IP and OP sampling pulses are derived for each oscillator cycle. The sampling time is short as compared with the cycle duration.

[0043] These pulses are fed one at a time through a selector switch (10) to a high-speed sample and hold amplifier (11). The input and output of the sample and hold amplifier along with sampling pulses are shown in timing diagram (FIG. 2).

[0044] The receiver module includes a series of electronic circuits to condition an analog signal detected by the receiver coil (12). The receiver coil has inductance of £=500 mH, SWG=42, diameter of the coil=5×10⁻³ m and number of turns=6500.

[0045] The receiver coil picks up resultant tune varying magnetic field due to transmitter setup and conducting target (16).

[0046] The receiver signal obtained from a coil placed and oriented as required with reference to the transmitter coil feeds a preamplifier (13) employing high input impedance FET operational amplifiers.

[0047] The output of the preamplifier is fed to a fourth order Butterworth high pass filter (14), to provide flat response which leads phase stability in the frequency range.

[0048] The output of the filter is amplified with a suitable variable gain amplifier (15). The amplifiers (13 and 15) are designed to optimize the signal to noise ratio over the frequency range.

[0049] The amplifier output is sampled with IP or OP sampling pulse through S/H amplifier. The S/H output is fed to a 12 bit analog to digital converter shown in FIG. 3.

(i) Digital Hardware

[0050] The system digital hardware and data processing unit is shown in FIG. 3 The processing system typically includes 8 bit 8085 microprocessor, memory 8 K EPROM, 16 K RAM, 48 I/O lines, 12 bit high speed analog to digital converter and a standard RS-232 C communication interface. The hardware has been designed around a standard EURO-bus structure so that functional enhancement of the system can be easily implemented. The measured parameters data is stored and displayed on a 2 line, 32 digit alphanumeric LCD display. Later the data is transferred to PC through RS-232 communication to obtain hardcopy of the profile data.

[0051] A high speed 12 bit A/D converter has very precise calibration and ensures obtaining good measurement results. The high computing ability of the system allows the quality of the data to be improved with selective stacking method.

[0052] The system measurement cycle timing diagram is shown in FIG. 2. First it checks the complete cycle of IP or OP sampling pulse and then issues data acquisition command to initiate digitization of the signal. The digital A/D data is stored and displayed.

[0053] At each step of T-R system, position the TP and OP components are measured stored and displayed on LCD display.

(ii) Software

[0054] The system operational software flow chart is shown in FIG. 4. The software (enclosed) is developed in assembly language includes:

[0055] a) data acquisition and averaging of samples to enhance signal to noise ratio

[0056] b) data computational software of anomalous IP and OP field components given as ${IP} = {\frac{{IP}_{P} - {IP}_{R}}{1P_{F}} \times 100}$ ${OP} = {\frac{{OP}_{F} - {OP}_{R}}{{IP}_{F}} \times 100}$

[0057] IP_(F) free space IP component of the primary field

[0058] OP_(P) free space OP component of the primary field is zero

[0059] IP_(R) IP component of the resultant field

[0060] OP_(R) conponent of the resutant field

[0061] c) presentation of measured data on LCD display

[0062] d) data transfer on to P.C.

Experimental Procedure

[0063] i) a variable frequency transmitter (frequency range 1 kHz to 15 kHz)

[0064] ii) a dipole-dipole transmitter-receiver coil configuration adjustable to several orientations, variable mutual separation and height above the model

[0065] iii) a resolved component measuring system—detected and recorded

[0066] iv) a large set of different metallic sheet ore models (aluminum, stainless steel, graphite) having different lengths, depths extents and thickness, a wooden model tank and tray for holding salt solution to simulate conducting host and overburden, respectively and

[0067] v) the transmitter and receiver coils are mounted on an adjustable frame which is fitted on a carriage moving over wooden rails. A scale fixed to one of these rails indicates the position of the coils. The rails are placed on top of a wooden tank containing the model conductor and salt solution (when simulating conducting host medium).

Operational Procedure

[0068] 1. Switch on multi output dc power supply 5 V, 15 V and −15 V

[0069] 2. Operate reset key on the keypad attached to the panel of the unit—Display shows “MICRO-85 ”

[0070] 3. Operate block move and shift keys—Display shows “No”

[0071] 4. Operate keys to display 6000—Display shows “6000.No”

[0072] 5. Operate key next—Display shows “”

[0073] 6. Operate keys to display 6 FFF—Display shows “6 FFF.No”

[0074] 7. Operate next key—display 6 FFF disappears “no” remains

[0075] 8. Operate keys to display 4000.—Display shows “4000.No”

[0076] 9. Operate execute key

[0077] 10. Switch on the function Generator and select “sinc wave” output

[0078] 11. Select frequency selector band switch for desired operating frequency

[0079] 12. Operate keys go—Display shows “go”

[0080] 13. Operate keys 4000 to display and operate execute key—Display shows “EM MODEL LAB WELCOMES YOU”

[0081] 14. Move T-R system away from the model, i.e., to one end of model tank

[0082] 15 Put selector switch in PRIMARY mode

[0083] 16. Put selector switch in IP measurement mode

[0084] 17. Put selector switch of receiver to ON

[0085] 18. Display shows “PRIMARY FIELD UP=______ mV”

[0086] 19. Adjust IP sampling time potentiometer to get maximum reading in display

[0087] Don't disturb this setting during the experiment

[0088] Set selector switch to OP measurement mode—Display shows “PRIMARY FIELD OP ______ mV”

[0089] 20. Adjust OP sampling time potentiometer to get minimum reading. Don't disturb this setting during the experiment

[0090] 21. Select switch from primary mode to anomaly mode

[0091] 22. Set selector switch in IP measurement

[0092] 23. Move T-R system tralley at a new location (2.5 cm) away from tank, wll operate measurement key—Display shows “ANOMALOUS FIELD IP=______%”

[0093] 24. Follow step No. 23 and at each location of T-R system observe and record IP values in the lab register. Also the data will be stored in the memory of the unit for transfer to a PC

[0094] 25. After completion of one profile bring back T-R tralley to a starting point

[0095] 26. Select a switch to OP measurement—Display shows “PRIMARY FIELD OP=______ mV”

[0096] 27. Select switch anomaly mode display shows “ANOMALOUS FIELD OP=______ %”

[0097] 28. Move T-R tralley across target and as in steps No. 23-25

[0098] 29. Set switch to primary mode. Operate reset key—Display shows “MICRO 85”

BRIEF DESCRIPTION OF THE ACCOMPANIED DRAWINGS

[0099]FIG. 1 is a simplified analog hardware functional blocks of electromagnetic measurement system.

[0100]FIG. 2 is a basic tuning diagram of the system.

[0101]FIG. 3 is a simplified digital hardware functional blocks of electromagnetic measurement system.

[0102]FIG. 4(a) Flowchart of measurement cycle.

[0103]FIG. 4(b) Flowchart of measurement cycle.

[0104]FIG. 5 presents comparative anomaly profile over a vertical sheet conductor for (A) zero field and equivalent (B) conventional horizontal coplanar coil system. It may be noted for the later, the anomaly is about 1.3% only whereas for the new system the anomaly is about 25%.

[0105]FIG. 6 presents anomaly profiles for the zero field, horizontal coil system for a conductor at different depths. Even for target depths of 15.6 cm the total anomaly is about 6%. For equivalent conventional EM system, a conductor deeper than 9.0 cm does not produce measurable response.

[0106]FIG. 7 presents comparative anomaly profile over a vertical sheet conductor for (A) zero field and equivalent (B) conventional Vertical coplanar coil system. It may be noted for the later, the anomaly is about 2.16% only whereas for the new system the anomaly is about 187%.

[0107]FIG. 8 shows anomaly profiles for zero field vertical coil system over model conductors at different depths. Compared to zero field horizontal coil system (FIG. 6) the vertical coil system records larger anomalies. 

1. A moving dipole electromagnetic modeling device for carrying out geophysical surveys to locate deeper and poorer electrically conducted subsurface bodies and for mapping subsurfacial conductivity changes due to salinity or pollution, and device comprising a combination of (i) analogue and (ii) digital systems: (i) said analogue system of electronic assembly of transmitter and receiver components comprising: (a) an oscillator (1); (b) power amplifier (2); (c) a transmitter coil (3); (d) tuning capacitors (4 & 5); (e) amplifiers (6 a & 6 b); (f) multitum potentiometers (7 a & 7 b); (g) dual comparator (8); (h) sampling pulse generator (9); (i) selector switch (10); (j) sample and hold amplifier (11); (k) receiver coil (12); (l) preamplifier (13); (m) filters (14); and (n) variable gain amplifier (15); (ii) said digital system of hardware and data processing unit comprising: (a) a microprocessor; (b) a storage means; (c) analog to digital converter; (d) a communication interface for generating output; and (e) means for data acquisition and data computation.
 2. A device according to claim 1, wherein the oscillator generates variable frequency from 1 kHz to 100 kHz sinusoidal signal with constancy of signal amplitude and frequency.
 3. A device according to claim 1, wherein the power amplifier is 10 watts assembly board receiving output from the oscillator (1).
 4. A device according to claim 1, wherein the transmitter coil generates primary electro magnetic field.
 5. A device according to claim 1, wherein the tuning capacitors consisting of an array of capacitors that resonantly tune the transmitter coil to achieve maximum output at desired frequency and to obtain nearly in phase (IP) sinusoidal signal with the transmitting current from the series resonant circuit.
 6. A device according to claim 1, wherein the capacitor network (5) produces nearly out of phase (OP) sinusoidal signal.
 7. A device according to claim 1, wherein the both IP and OP signals are fed to two separate amplifiers 6 a and 6 b.
 8. A device according to claim 1, wherein the input comparator (8) receives two amplified signals from two separate amplifiers 6 a and 6 b.
 9. A device according to claim 1, wherein the input comparator (9) also receives dc voltage from two multimeter potentiometers 7 a and 7 b.
 10. A device according to claim 1, wherein sampling pulse generator consists of two monostable multivibrators to generate 24 μs short duration sampling pulse.
 11. A device according to claim 1, wherein the hold amplifier (11) receives the sampling pulse through the selector switch (10).
 12. A device according to claim 1, wherein the receiver coil (12) conditions the analogue signal detected.
 13. A device according to claim 1, wherein the amplifier (13) receives output from the receiver coil.
 14. A device according to claim 1, wherein the high pass filters (14) provides a flat response to achieve the phase stability in the frequency range.
 15. A device according to claim 1, wherein the variable gain amplifier (15) receiving output from the fibers is designed to optimize the signal to noise ratio over the frequency.
 16. A device according to claim 1, wherein the receiver coil picks up resultant time varying magnetic field due to transmitter setup and conducting target (16).
 17. A device according to claim 1, wherein the digital hardware is designed around EURO-bus architecture to implement the functional enhancement.
 18. A device according to claim 1, wherein the high-speed A/D converter is used for precise calibration and to obtain good measurement results.
 19. A device according to claim 1, wherein said device having means to average the samples to enhance signal to noise ratio.
 20. A device according to claim 1, wherein the receiver coil is placed parallel to the transmitter coil at a point, where the primary field induces zero current (ZERO FIELD) to pick up only anomalous field.
 21. A device according to claim 1, wherein the device is also used to explore subsurface minerals, buried pipes and cables, landmines and archaeological artifacts.
 22. A device according to claim 1, wherein the depth of investigation of the device is about 75% more compared to that of similar conventional moving dipole electro magnetic systems.
 23. A device according to claim 1, wherein the device is also be used to measure IP and OP components of the anomalous field as in conventional EM methods such as horizontal coplanar, vertical co-axial and asymmetric configurations.
 24. A device according to claim 1, wherein by increasing the transmitting power the anomalies are increased thereby enabling the exploration of deeper and poorer electrically conducted sub-surface bodies.
 25. A device according to claim 1, wherein the said device is compatible with any Transmitter-Receiver coil configuration.
 26. A device according to claim 1, wherein the depth of investigation of the present device is enhanced by about 1.5 times.
 27. A device according to claim 1, wherein the total amplitude of the feeble secondary field produced by the target conductor measured in the absence of the large primary field is profitably utilized to locate deeper and poorer conducting zones even from airborne E.M. surveys.
 28. A device according to claim 1, wherein the same device is also used for configurations selected from Horizontal Coplanar, Vertical co-axial and asymmetric configurations.
 29. A device according to claim 1, wherein the devices used to measure amplitude of secondary field.
 30. A device according to claim 1, wherein said device having a transmitter coil placed strategically, which is not affected by the primary field and measures only the secondary field.
 31. An analogue sum digitally implemented method for carrying out geophysical surveys, by suing the device as claimed in claim 1, to locate deeper and poorer conductors and for mapping subsurfacial conductivity changes due to salinity or pollution, said method comprising the steps of: (a) measuring the anomalous field (resultant of primary and secondary field due to conducting target) picked up by a receiver coil and resolved into in phase (IP) and out of phase (OP) component using a sampling technique; (b) deriving IP and OP sampling pulses from transmitter coil current for each transmitting current cycle to measure as said in step (a) with the sampling time is short (2 μs) compared to transmitting cycle time; (c) measuring the resultant electro magnetic field data as step (a) and digitizing through A/D converter at the same point for about one hundred samples; (d) averaging the data to enhance signal to noise ratio to achieve a high degree of accuracy; (e) computing in phase (IP) and Out of phase (OP) components of anomalous field and storing in binary data form; and (f) displaying on an output device and transferring the data to a digital computer through a communication interface.
 32. The method according to claim 24, wherein the total amplitude of the feeble secondary field produced by the target measured in the absence of the large primary field is profitably utilized to locate deeper and poorer conducting zones even from airborne EM survey.
 33. The method according to claim 24, wherein the amplitude of the secondary field due to poorer conductor is increased by increasing power of the transmitter coil.
 34. The method according to claim 24, wherein the secondary field amplitude is increased by increasing transmitter signal frequency. 